1. Technical Field of the Invention
This invention relates to telecommunication systems. More particularly, and not by way of limitation, the present invention is directed to a method of optimizing hysteresis values utilized for automated handoff of mobile stations operating in a cellular radio telecommunications network.
2. Description of Related Art
In cellular radio telecommunications networks, as a mobile station (MS) moves away from its serving base station, the received signal strength at the MS decreases while the received signal strength from a neighbor cell increases. Eventually, when the signal strength from the neighbor cell exceeds the signal strength from the serving cell, a handoff is performed. Under certain circumstances, the MS may be undesirably handed off back and forth in a fairly rapid manner between two or more cells. This phenomenon is known as handoff oscillations, and has many adverse effects in the cellular network. First, temporary muting is experienced by the subscriber during each handoff, and this adversely affects voice quality when it is repeatedly experienced in a short period of time. Second, the probability of dropping the call is cumulatively increased since the probability of dropping a call is increased during each handoff. Third, the repeated handoffs add to the processor load in the network.
One of the main causes of handoff oscillations is the manner in which MSs obtain signal strength measurements from the serving cell and candidate neighboring cells. Erroneous signal strength measurements from neighbor cells can be caused by a design defect in certain MSs, by poor cell planning causing co-channel interference, or poor propagation in certain areas. In general, the received signal strength decreases in a log-normal fashion as the MS recedes from the serving base station. Variations in the received signal strength are superimposed on the log-normal decrease. These variations may be caused by multi-path constructive and destructive interference (long-term fading) and Rayleigh (short-term) fading.
Due to the signal strength variations, MSs often do not report signal strengths that are proportional solely to the MSs"" distances from the serving base station and neighboring base stations. The variations may make the signal strength from the neighboring base station appear better than the MS""s position would dictate, and better than the signal strength in the serving cell. The handoff is then performed based on the erroneous measurements. This places the MS in a new serving cell where the measurements then show that the signal strength is better in the old serving cell. Therefore, another handoff is performed back to the old serving cell.
In previous solutions to the handoff oscillation problem, an automated hysteresis value is calculated and utilized to control the rate of oscillating handoffs. The hysteresis value may be calculated for a particular cell based on the standard deviation of the received signal strength reported by, or oscillation rate experienced by, an MS in the cell. In order for an outgoing handoff to be performed, the received signal strength from a candidate neighbor cell must exceed the signal strength of the serving cell by the hysteresis amount. Once the MS is handed off, the application of the hysteresis value is reversed, and the signal strength from the original serving cell must exceed the signal strength of the new serving cell by the hysteresis amount before a handoff back will be performed. A hysteresis margin may be added to the normal hysteresis value to reduce the probability that the MS will be handed off back to the original serving cell.
However, every MS is affected in the particular cell when the automated hysteresis value is determined and set. If the hysteresis value is not optimized, the network""s handoff performance is adversely affected. For MSs experiencing severe handoff oscillation problems, this trade-off may be acceptable. However, all MSs are not uniformly affected by oscillating handoffs. This creates unbalanced handoff borders and excessive interference, requiring that the value of the hysteresis be set higher, thereby further degrading handoff performance for all MSs in the cell.
In order to overcome the disadvantage of existing solutions, it would be advantageous to have a method of optimizing the automated hysteresis value in a cellular network. The present invention provides such a system and method.
In one aspect, the present invention is directed to a method in a cellular telecommunications network for optimizing a handoff border between a present cell and a neighbor cell. The method optimizes a current hysteresis value utilized to ensure that a mobile station (MS) is receiving a stronger signal from the neighbor cell than the present cell before a handoff is performed from the present cell to the neighbor cell. Pairs of signal strength (SS) measurements are collected from a plurality of MSs operating near the handoff border. Each SS measurement pair includes an SS measurement from the present cell prior to a handoff and an SS measurement from the neighbor cell following the handoff. A midpoint SS is then calculated by determining an average SS measurement from the present cell, an average SS measurement from the neighbor cell, and an overall average of the average SS measurement from the present cell and the average SS measurement from the neighbor cell. The method then optimizes the handoff border by subtracting from the current hysteresis value, an amount approximately equal to the difference between the midpoint SS and the average SS measurement from the present cell.
In another aspect, the present invention is directed to a method of optimizing an outgoing hysteresis value utilized for controlling handoff of an MS from a present cell to a neighbor cell at a handoff border in a cellular telecommunications network. The method includes the steps of determining a current outgoing hysteresis value, and collecting a plurality of SS measurements taken by a plurality of MSs operating near the handoff border. The SS measurements include, for each mobile station, a first SS measurement of a signal received by the mobile station from a first base station serving the present cell, and a second SS measurement of a signal received by the mobile station from a second base station serving the neighbor cell. The SS measurements are then utilized to calculate an average SS on the present cell side of the handoff border (CP), and an average SS on the neighbor cell side of the handoff border (CN). A midpoint SS between the present cell and the neighbor cell is then calculated by calculating an average of CP and CN. Next, an expected signal strength on the present cell side of the handoff border following the handoff (EP) is calculated by subtracting a factor due to rounding errors from the midpoint SS. Finally, a recommended hysteresis value for outgoing handoffs from the present cell to the neighbor cell is calculated by subtracting from the current outgoing hysteresis value, the difference between EP and CP.
In yet another aspect, the present invention is directed to a method in a cellular telecommunications network for optimizing an incoming hysteresis value utilized for controlling handoff of a mobile station from a neighbor cell to a present cell at a handoff border. The method includes the steps of determining a current incoming hysteresis value, and collecting a plurality of SS measurements taken by a plurality of MSs operating near the handoff border. The SS measurements include, for each mobile station, a first SS measurement of a signal received by the mobile station from a first base station serving the present cell, and a second SS measurement of a signal received by the mobile station from a second base station serving the neighbor cell. The SS measurements are then utilized to calculate an average SS on the present cell side of the handoff border (CP), and an average SS on the neighbor cell side of the handoff border (CN). A midpoint SS between the present cell and the neighbor cell is then calculated by calculating an average of CP and CN. Next, an expected signal strength on the neighbor cell side of the handoff border following the handoff (EN) is calculated by adding a factor due to rounding errors, and adding an MS output-power step-size factor to the midpoint SS. Finally, a recommended hysteresis value for incoming handoffs from the neighbor cell to the present cell is calculated by subtracting from the current incoming hysteresis value, the difference between EN and CN.